1. Field of the Invention
The present invention relates to an optical pickup device, which has an objective lens with a high NA, for recording and reproducing information onto and from an optical information recording medium.
2. Description of the Background Art
Since light-employing techniques enable a large number of features such as high speed processing due to high frequencies, spatial information processing and phase processing, they are subject to research and development and put to practical use in wide-ranging areas including communication, measurement and processing.
For the techniques, high-precision objective lenses are used to reduce optical beams.
In recent years, expectations for image recording devices, particularly those employing light have been high, and techniques for mass storage capacity have become important. In addition to the importance of improving a recording medium, it is essential for mass storage of optical information recording to provide a smaller radius of a beam spot, that is, to sufficiently narrow down a beam spot by using an objective lens.
As is well known, a beam spot radius is proportional to a light wavelength and inversely proportional to an objective lens NA (Numerical Aperture). For the wavelength, blue laser diodes or blue or green SHG lasers have been developed in recent years. For the higher NA of an objective lens, higher density has been achieved such that the NA is 0.6 for DVDs (Digital Versatile Disks) as compared with 0.45 for CDs (Compact Disks). However, attaining much higher NAs has been difficult from a manufacturing point of view by using a single lens with two aspheric surfaces. Therefore, an optical pickup device for attaining a higher NA by using two-group, two-element lens system is disclosed in Japanese Patent Laying-Open No. 10-123410.
FIG. 11 shows a 2-group 2-element objective lens disclosed in Japanese Patent Laying-Open No. 10-123410. An objective lens 101 is formed of a first lens 102 having two aspheric surfaces and a second lens 103 of a plano-convex lens having an aspheric shape for the convex surface, and the objective lens has an NA of 0.85. Light which passes through objective lens 101 enters a transparent body 104 and forms a beam spot at an image point 105.
However, in an optical pickup device having a high NA objective lens, the inventors found out that the shape of a beam spot becomes oval due to the polarized state of incident light, resulting in bad effects on jitter and crosstalk characteristics, for example.
In order to solve the above described problems, an object of the present invention is to provide an optical pickup device which can prevent deterioration of the jitter and crosstalk characteristics, for example, due to oval deformation of a beam spot.
An optical pickup device for attaining the above object according to the present invention collects a light beam from a light source by an objective lens and irradiates an optical recording medium. In the optical pickup device, the objective lens has a numerical aperture (NA) of 0.7 or more, and the ellipticity of polarized light of the light beam which is incident upon the objective lens is set to be larger than (1.4xc3x97NA)xe2x88x920.7.
By setting the numerical aperture of the objective lens and the ellipticity of polarized light of the light beam which is incident upon the objective lens to satisfy the above condition, it is possible to improve the jitter characteristic and reduce crosstalk.
In a preferred embodiment of the optical pickup device according to the present invention, the light beam which is incident upon the objective lens is elliptically polarized light, and the longer axis direction of the ellipse is almost parallel to the track width direction of the optical recording medium.
Thus, the beam spot radius in the track direction can be set as small as possible and, as a result, the bad effects on the jitter characteristic can be suppressed.
In the optical pickup device according to the present invention, the objective lens can be formed of two lenses arranged on an identical optical axis.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.